The present invention relates generally to a system and method for magnetic resonance (MR) imaging of hyperpolarized substances and, more particularly, to a method of decreasing undesirable effects of proton coupling on available signal-to-noise ratio (SNR) for hyperpolarized contrast agent imaging.
When substances such as human tissue or contrast agents are subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the substances attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If a substance, contrast agent, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The set of received nuclear magnetic resonance (NMR) signals resulting from a scan sequence is digitized and sent to a data processing unit for image reconstruction using one of many well known reconstruction techniques.
Imaging with MR contrast agents can be done in multiple ways. Certain substances, known as paramagnetic contrast agents, increase the magnetization and/or polarization of surrounding substances, and are therefore not themselves a source of MR signals. Other contrast agents contain excitable non-hydrogen nuclei, such as 13C, 14N, 31P, 19F, and 23Na, which produce their own MR signals, rather than increasing MR signal strength of surrounding tissues. Several methods of enriching and hyperpolarizing such substances have been developed to further increase signal strength and imagability thereof.
One drawback of conventional methods of imaging non-hydrogen nuclei is the effect that spin interactions can have on available SNR. For example, the coupling between spins of a hydrogen proton and a directly bonded carbon-13 isotope can cause resonance frequency splitting. Thus, the spectral profile of a substance having a bonded or “protonated” carbon will appear wider and weaker. This splitting generally results in a spectral profile having a number of “peaks” of varying and predictable strength, proportionate to the coupling constant of the interacting spins of the substance. Accordingly, resolution of an image can be affected when resonant frequencies to be imaged are nearby, since spectral profiles can overlap, cancel, or enhance one another.
One method of overcoming this drawback in substances having protonated carbons (or other imagable nuclei) is known as proton decoupling. These methods typically include the use of a saturating B1 excitation field to reduce or eliminate the effect of proton spins on the resonance of other excitable nuclei of interest. Most non-hyperpolarized 13C applications utilize such an approach. Since the relevant spectra of these compounds can be relatively close, proton decoupling is used to provide an increase in image resolution. However, these methods are not known to be used in imaging of hyperpolarized substances or substances with non-protonated nuclei of interest. When imaging hyperpolarized substances, sharply declining free induction decay (FID) signal strengths and FID signals of lowered initial strength can limit available sampling time and SNR. Additionally, because of the susceptibility of hyperpolarization to destruction from RF pulses, increasing flip angles may not adequately compensate for reduced sampling time or SNR.
It would therefore be desirable to have a system and method which overcomes the aforementioned drawbacks of non-hydrogen and hyperpolarized imaging. In particular, it would be desirable for such a system and method to improve T2* decay rate and signal strength of hyperpolarized substances for increased available sampling time and SNR.